Such a loom rotor is shown in Steiner U.S. Pat. No. 4,586,541. A fixed switching tube having apertures in the shell surface closely abuts the cylindrical surface of a rotor and supplies air into bore holes in the cylindrical shell rotating past. The apertures in the switching tube lie on a helical line so that a travelling field is produced via the bore holes associated with one respective row of reeds, to which the relay system jets are connected. A disadvantage of this arrangement is that the travelling field is predetermined by the geometry of the switching tube. The maximum possible velocity for weft insertion can only be used with a determined loom width. With loom widths smaller than this, unused pauses between the individual weft beatups ups are produced with the maximum possible insertion velocity. The invention solves this problem.